Process for sweetening sour hydrocarbons



United States Patent O 3,153,628 PRCESS FR SWEETENENG SUR HYDROCARBNS Robert A. Moon, fir., Puerto La Cruz, `Veneruela, assigner to Gulf Oil Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Dec. 29, 1961, Ser. No. 1163,331 Ciaims. (Cl. 208-193) This invention relates to a modified doctor-sweetening process, and more particularly to a two-stage sodium plumbite sweetening process for sour hydrocarbon distillates.

Light petroleum distillate oils, particularly catalytically cracked gasoline, are frequently unmarketable as manufactured because of their mercaptan sulfur content, as even a few thousandths percent mercaptan sulfur can impart a disagreeable odor to the oil and render the oil sour to the doctor test. In the case of gasoline, mercaptan sulfur also can reduce oxidation stability and lead susceptibility. As a consequence, sour, or mercapstan-containing, light oils are ordinarily subjected to some sort of sweetening treatment prior to marketing. Perhaps the best known of such treatments is the doctor sweetening process. In its conventional form, the doctor sweetening process involves treating sour distillate with a small amount of elemental sulfur and an aqueous sodium plumbite, or doctor solution, whereby the mercaptans are converted to disuldes, and the sodium plumbite is converted to sodium hydroxide and lead sulfide.

Notwithstanding its long usage, the conventional doctorsweetening process is subject to certain disadvantages. For example, direct treatment of raw, sour oil with doctor solution may involve an appreciable consumption of lead oxide, a relatively expensive chemical. Moreover, when the sour oil contains appreciable amounts of hydrogen sulfide, a significant proportion of the plumbite reagent may be consumed simply by reaction therewith, prior to removal of any mercaptans. Consumption of lead oxide has been minimized in some instances by regeneration of spent or partially spent doctor solution, and by pretreatment of the raw oil with aqueous caustic soda solution. However, pretreatment of the sour oil with caustic solutions is not entirely satisfactory, as additional, separate, solution-contacting and storage facilities are required. Regeneration of spent doctor solution is not always fully satisfactory, particularly in the case of cracked distillates, as the spent treating solution may be contaminated with sodium salts of phenolates and other organic acids in the distillates. These salts are not removed by regeneration of the doctor solution and tend to promote stable emulsions, especially in the presence of strong caustic solutions. Emulsiiication, in turn, leads to oil losses, excessive settling times, discoloration of treated oil, and reduced additive response. Moreover, regeneration of spent doctor solution requires additional treating vessels, pumps, valves, etc.

Another disadvantage of doctor sweetening arises from the addition of elemental sulfur to the oil. lf the amount of sulfur added is greater than needed, the excess is carried over into the product, either as such or in the form of organic polysulfides, thus rendering the treated oil corrosive and/or decreasing its resistance to oxidation and reducing its response to oxidation inhibitors and tetraethyl lead. On the other hand, addition of too little elemental sulfur may result in the carryover of mercaptans, in the form of oil-soluble lead mercaptides, into the finished oil.

It has also been proposed to sweeten sour hydrocarbon oil distillates by the so-called lead sulfide process, which involves contact with aqueous caustic soda, lead sulfide, and a small amount of oxygen or air, but

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such treatment has not always been found entirely satisfactory in that it may result in sweetened oils having a relatively poor response to oxidation inhibitors.

The present invention relates to a modified doctorsweetening process in which consumption of lead oxide is minimized without regeneration of spent plumbite solution, in which severe emulsication diiculties are avoided, in which difficulties arising out of the addition of free sulfur are avoided, and in which the sweetened products have good response to oxidation inhibitors and tetraethyl lead. It has now been found that sour, light hydrocarbon oil distillates can be sweentened by intimately contacting previously unsweetned sour distillate concurrently with a small amount of oxygen, either as such or in the form of a mixture such as air, said small amount being at least chemically sufficient to oxidize the mercaptan sulfur in the oil, and with partly spent, aqueous, lead sulfide-containing, sodium plumbite solution, separating the treated oil and partly spent sodium plumbite solution, intimately contacting the thus-separated oil with a second aqueous sodium plumbite solution that is substantially free from lead sulfide and that is of greater strength than the partly spent solution, separating sweetened oil from said second sodium plumbite solution, and either continuously or from time to time replacing at least a portion of said partly spent sodium plumbite solution with a like amount of said second sodium plumbite solution, and replacing the amount of the second sodium plumbite solution so Withdrawn with a like amount of fresh sodium plumbite solution of the original strength.

In the single figure of drawing there is shown a simplitied ilow diagram of apparatus in which the process of the present invention can be carried out.

The partly spent sodium plumbite solution employed in the initial treating stage of the process is a complex mixture containing, among other things, caustic soda, lead sulfide, and sodium plumbite. The strength of the solutionwith respect to caustic soda, lead oxide, and lead sulfide is not especially critical, as these materials are only slowly consumed in the process and, to a large extent, regenerated in the sweetening reaction itself. We prefer to employ rather strong, partly spent, sodium plumbite solutions, as they are generally more effective, and the volume of solution to be handled is thereby reduced substantially. Excellent results have been obtained by the use of partly spent aqueous sodium plumbite solutions containing about 19 percent sodium hydroxide and that are substantially saturated with lead oxide at ambient atmospheric temperatures, but any other partly spent sodium plumbite solutions of a strength normally employed in the doctorsweetening process can be used. Thus, there can be used partly spent sodium plumbite solutions containing about 4 to 24 percent by weight sodium hydroxide and about 0.5 to 5 percent by Weight lead oxide. Larger proportions of lead oxide can be dissolved in the caustic solution at elevated temperatures.

The amount of lead sulfide in the solution is not critical, as this material is not substantially consumed in the process when all of the operating variables are properly controlled. The lead sulfide in the treating solution is obtained by reaction of lead oxide or sodium plumbite in the treating solution with hydrogen sulfide in the sour stock and possibly to some extent from the sodium plumbite solution transferred from the second stage of the process to the rst stage. Alternatively, lead sulfide can be added from an external source.

Best results are obtained when the quantity of air or oxygen employed in the first stage of the process is kept small. The amount of oxygen, in whatever form it may be employed, should be at least stoichiometrically equivalent to the quantity of mercaptan sulfur in the sour oil, that is, chemically sufficient to oxidize the mercaptan sulfur in the oil, that is, at least in a 1:4 mol ratio with the mercaptan sulfur, and preferably somewhat in excess of this amount, say, two to four times the minimum amount. However, the amount of oxygen employed should not be so large as to oxidize the lead sulfide in the solution at a faster rate than it is introduced or formed therein. For typical oils, good results are obtainable by the use of 0.l5 to 1.0 cubic foot of air per barrel, the optimum amount varying with the quantity of mercaptans in the sour oil, but greater or lesser amounts can be used in appropriate instances. The use of oxygen or air is critical to the process as its absence can lead to the production of sweetened stocks that do not remain doctorsweet on standing. The air can be added to the sour oil before, during, or after admixture with the partly spent sodium plumbite solution, so long as the contact of the 'sour oil and both reactants is concurrent for a period of time suflicient to sweeten the oil.

The conditions employed in the iirst stage of the herein-disclosed sweetening process can be any normally employed in doctor-sweetening of sour oils. Thus, while the sweetening reactions undoubtedly proceed to some degree at ambient atmospheric temperatures, elevated temperatures are preferably employed to accelerate the reaction and reduce settling time. By way of example, there can be used with good results temperatures in the range of 60 to 175 F., depending on the nature of the stocks, and even higher temperatures, up to 260 F. can be employed in closed system maintained under pressure. When treating cracked sour stocks, temperatures less than 120 F., preferably in the range of about 85 to 100 F., are desirable in order to minimize oxidation of olefins in the cracked stock by the air or oxygen introduced into the system. Pressures of about to 75 p.s.i.g. can be used to advantage with elevated temperatures in order to prevent volatilization of the oil during the process.

The total time of contact between the air or oxygen, lead suliide, and partly spent sodium plumbite solution can vary within wide limits depending upon the nature of the mercaptans in the oil, the reaction temperature, the viscosity of the oil at that temperature, the intimacy of contact between the respective liquids, and the concentration of the reactants in the treating solution. Within the limits normally employed in doctor-sweetening reactions with respect to these factors, Contact times and treating times inthe range of a few seconds .to eight hours can be used. In the instance of non-viscous light oils such as gasoline, the total contact time for the bulk of the treated oil may be only the few seconds required for mixing and transporting to the settling vessel, as substantial separation into distinct phases occurs quickly in the settler. For heavier oils and other mixing methods, mixing times of as long as 10 minutes may be employed to obtain the desired degree of mixing, and in these instances, contact time can also be prolonged by the necessity for longer settling times. As a rule, relatively shorter settling times are requiredfor stocks and treating conditions that do not promote stable emulsions, and relatively longer times are required for stocks and conditions tending to promote relatively more stable emulsions.

The treating solution can be contacted with the sour oil in any proportion normally used in the doctor-sweetening process. Within the usual practical limits of intimacy of contact between oil and aqueous phases and settling time, good results will be obtained by the use of treating solution in the proportion of about l0 to 100 percent, and preferably about to 60 percent, by volume of the sour oil.

The sodium plumbite solution employed in the second stage of the process is substantially stronger than the first stage Vplumbite solution with respect to caustic soda and lead oxide content, in order that transfer of solution from the second stage to the first will restore the solution in the iirst Vstage to the selected, optimum strength, and

for other reasons hereinafter described. As a matter of fact, because only a small amount of lead suliide is formed in the present process and because of the preliminary contact of sour oil and caustic in the iirst stage, emulsion difficulties are minimized in the second stage of the process, and relatively more concentrated sodium plumbite solutions can be employed in the second stage than might be customarily used in the doctor-sweetening process, with all the advantages normally attending the use of such stronger solutions. Thus, excellent results are obtainable by the use of 40 B. (approximately 35 percent by weight) caustic soda solutions containing about 3 percent lead oxide in the sweetening of cracked gasoline, but solutions containing other appropriate amounts of sodium plumbite normally used in the doctor-sweetening process can also be used. By way of illustration, there also can be used conveniently in the second stage of the process aqueous sodium plumbite solutions containing about 7 to 40 percent by weight sodium hydroxide and about l to 5 percent by weight lead oxide, so long as they are substantially stronger. Excellent results have been obtained when the fresh, second stage, sodium plumbite solution contains about percent more caustic soda and lead oxide than the partly spent solution employed in the first stage of the process, but solutions of greater or lesser difference in strength can be used. Thus, there can be used in the second stage fresh sodium plumbite solutions containing 25 to 200 percent more caustic soda and sodium plumbite than the solution used in the first stage.

The treating temperature, contact time, settling time, ratio of treating solution to oil, and other treating conditions in the second stage are governed by the same factors dealt with above in connection with the first treating stage.

With the passage of time, the partly spent sodium plumbite solution employed in the iirst stage of the process becomes contaminated with sodiumphenolates-which facilitate extraction of mercaptans-'sodium salts of other carboxylic acids, lead sulfate and other materials, and the solution tends to become more dilute. Also, the solution employed in the second stage treatment tends to become less effective to achieve the desired stability and antioxidant response in the finished product, presumably as a result of the extraction of phenolic materials that are not removed by the rst stage solution. These ditiiculties are overcome by periodic or continuous removal, preferably the latter, of a portion of the partly spent s0- dium plumbite solution employed in the rst stage of the process and by replacement thereof with a like amount of sodium plumbite solution from the second stage of the process, and by introduction of fresh make-up sodium plumbite solution into the second stage, so as to'restore or maintain the desired solution strength in both stages. The amount of solution that should be withdrawn from and replaced in the system will vary principally withithe amount of oxygen introduced into the system, greater -roportions being withdrawn and replaced with increasing mercaptan sulfur and phenolic content and with increasing oxygen injection. For oils containing about 0.01 to 0.015 percent mercaptan sulfur and for ai'r injection in the proportions disclosed herein, we have found it convenient to withdraw and replace about 10 percent of the solution inventory per day in each stage of the process. When other oils of different mercaptan sulfur content are employed, amounts in the range of about 5 to 25 percent or more of the solution inventory in the respective stages can be withdrawn and replaced. insofar as extraction of phenolic materials is concerned, the process in principle can be operated with 100 percent replacement of the second stage treating solution each day. We have found that more economical antioxidant consumption is achieved by replacement of solution in the range of amounts indicated, and this procedure is therefore preferred.

It has been found that in the case of gasoline a convenient index as to when and to what extent solution should be withdrawn and replaced is the oxygen stability and the tendency to form gum in the copper dish test by the sweetened product. The copper dish gum content of the treated product in particular tends to rise to an unsatisfactory level, that is, above mg./ 100 ml., more quickly than the ability of the solutions to sweeten the gasoline tends to decline. Thus, when the solutions in both stages are at proper operating equilibrium, response is good, that is, the amount of inhibitor required to achieve a given level of stability will be relatively small. As the solutions continue to be used, the inhibitor response of the product tends to decline. When the inhibitor response reaches an objectionable level, as say, when the cost of the additional inhibitor required to stabilize the sweetened product becomes greater than the cost of the malte-up, fresh sodium plumbite solution, the solutions should be withdrawn and replaced as described. By way of emphasizing the importance of achieving optimum operating equilibrium, it has been found that the oxygen stability of the finished product is not as good when the second stage solution is entirely fresh, as it is after the process has operated for a while. Accordingly, it is advantageous to withdraw and replace only a fraction of the second stage treating solution each day.

The process of this invention is useful in the sweetening of any mercaptan-containing hydrocarbon oil distillate. The process is especially useful with the light or lower-boiling distillates such as gasoline. Excellent results have been obtained in the sweentening of sour, fluid catalytically cracked gasoline distillates, but other sour distillate stocks can also be treated with success using the process of this invention. Examples of such other stocks are straight-run gasoline, naphtha, kerosene, jet fuel, diesel fuel, furnace oil, gas oil and the like. The stocks to be treated are subjected to the herein-disclosed process without any sweetening pretreatment, and they are preferably treated When they are freshly prepared, that is, while they are still at an elevated temperature following distillation or other refining processes involving heating of the stocks.

The chemical reactions that take place in the initial sweetening stage are complex and therefore somewhat obscure. The fundamental effect, of course, may be represented in simplified form by the following equation:

Although the invention is not limited to any particular theory, the detailed reaction sequence may be as follows:

From the foregoing series of reactions it will be seen that the lead sulfide consumed in the process is regenerated during sweetening. This is advantageous, as accordingly, only a relatively small amount of lead sulfide need be present in the process. Emulsification difiiculties are thereby minimized. In contrast, in ordinary doctor-sweetening processes, lead sulfide is continuously produced in a fixed ratio with the mercaptan sulfur content. It will also be noted that in the present process, the elemental sulfur consurned in the sweetening reaction is generated during the process. Elemental sulfur may also be furnished in part by the sour stock itself. The lead suliiide may be introduced into the solution in the first instance by reaction of H28 in the sour oil with sodium plumbite:

(VI) NaZPbOZ-l-HzSe ZNaOH-i-PbS Alternatively, or in addition, lead sulfide may be supplied to the first stage by the second stage treating solution.

Because of the sodium hydroxide content of the rst stage treating solution, the following reactions may also occur in such stage in greater or lesser degree:

(VIII) PbS In addition, side reactions involving extraction of phenols, including cresols and xylenols, naphthenic acids and the like can take place in the initial stage.

The functioning of the fresh sodium plumbite solution in the second stage of the process is relatively more obscure. To some extent, the second stage sodium plumbite solution probably acts to remove any mercaptans that remain in the oil by forming lead mercaptides that are then converted to lead sulfide and disulfides by reaction with any oxygen or sulfur carried over in the oil from the first stage, or by the following reaction:

However, the effect of the second stage treatment is considered only incidentally to involve additional sweetening of the oil.

The second stage sodium plumbite solution may also function in part by converting any lead mercaptides or lead sulfide'carried over in the oil from the first stage or formed in the second stage treatment to basic lead mercaptides in the first instance, and ultimately, in both instances, to basic lead sulfide, which is more readily separable from the oil. To the extent that it is formed, this material is transported to the first stage when the treating solution is advanced in the process, where it can act as a source for the lead sulfide required in the first stage.

Whether or not the second stage treating solution contributes to any part of the sweetening mechanism, it has been found in fact that such second stage treatment is advantageous in that it effectively minimizes the amount of oxidation inhibitor that is required to achieve a given level of stability in the sweetened oil. It is thought that this effect is brought about by further extraction of phenolic compounds and perhaps peroxides that are not extracted in the first stage because of insutiicient acidity or other reason. Although these compounds are considered to act as natural inhibitors, over a period of time they tend to promote gums. Accordingly, superior sta- -bility in the final product is obtained by removal of a portion of these trace materials in the second treating stage, following which they are replaced with a commercial inhibitor.

It is emphasized that the order of the treating steps and the absence of any prior sweetening pretreatment are important in the present process. Thus, by avoiding prior sweetening pretreatments, hydrogen sulfide is available in the first stage to reactwith sodium plumbite and produce the relatively small amount of lead sulfide that is necessary in the first stage reaction. On the other hand, by sweetening with lead sulfide in the first stage rather than with sodium plumbite, undue consumption of lead oxide in the process and the necessity for regeneration of spent solution are avoided. Also, the use of fresh, strong sodium plumbite solution in the second treating stage is important to minimize consumption of oxidation inhibitor in the finished product. By avoiding addition of elemental sulfur in either stage, corrosiveness of the sweetened product, poor lead response, consumption of lead oxide, and emulsification problems and product quality difliculties due to excessive lead sulfide formation are avoided or minimized.

Referring again to the drawing, uid catalytically cracked gasoline obtained from a fractionating tower having a mercaptan sulfur content of about 0.003 to 0.027 percent by weight, for example 0.015 percent, and containing a trace amount of H28 is introduced directly into the system at a temperature of about F. at the rate of 195 barrels per hour, by means of a pump, not shown, by way of line 4. The sour oil stream is joined by air in the amount of 1.2 mols per hour from yline 8 and by about 54.9 percent by volume of the lead sulfide-containing, partly spent, 18 to 25 B. sodium plumbite solution from line 6 that comprises the first stage treating solution. The mixture of sour oil, air, lead sullide, and partly spent sodium 'plumbite solution is passed into mixer 10 where intimate contact is obtained and after an estimated contact time of six seconds, to settler ft2, where the oil and aqueous phases are allowed to separate for an average time of about 1.3 hours. Partly sweetened oil, still at a temperature of about 100 F. and at this stage having relatively low oxygen stability and high gum content-passes from settler 12 to line 18 at the rate of 195 barrels per hou. where it is joined by about 30.8 percent by volume of the 40 B. sodium plumbite solution kfrom line 20 that comprises the second stage treating solution. From line 1S, the mixture ofpartly sweetened oil and sodium plumbite solution passes through mixer 212, where intimate contact is obtained and continued through line 19 for a total of about 20 seconds, and thence to settler 24, where the oil and aqueous phases are allowed to separate for an average of about 1.3 hours. Sweetened gasoline is then passief vfrom settler 24 through line 29 to mixer .31 Where it is mixed with wash water from line 32, and thence to settler 33 for settling. The once-washed and settled gasoline passes from settler 33 through line 3d to mixer 3d where it is admixed with wash water from line 37. Inhibitors, such as metal deactivators and antioxidants, are conveniently introduced in the desired proportions, just prior to the second washing, 'oy way of line 38. The mixture of wash water and sweetened gasoline passes from mixer 36 through line 3% to settler 40 for iinal settling. The twicewashed and settled sweetened gasoline, containing less than 0.003 percent mercaptan sulfur, passes out of the system by way of line 4l.

The settled, lirst stage treating solution is recycled from settler Y12 through line 14, pump le and line to line 4. Settled, second stage treating solution is recycled from settler 24 through line 2o, pump 28, and line 2.0 to line jd.

Gnce daily, about l percent of the first stage treating solution inventory, approximately 1100 gallons (147 cu. ft.) isdrawn off from settler 12 and discarded. This solution is replaced with a like amount of the 40 B. sodium plumbite solution from settler 24 through line 35. The inventory of the second stage treating solution is reestablished by addition of 110 gallons of fresh, 40 Be.

`sodium plumbite solution-prepared by dissolving 436.6 lbs.

gum-content of less than 25 nig/100 ml. and an ASTM oxidation induction period of greater than 300 minutes after addition of about to 18 pounds di-tert-butyl-pcresol as an oxidation inhibitor. A typical sample of a gasoline sweetened and stabilized similarly as described had an EL-2 -test total engine cleanliness rating of 73 (100 perfect) and a piston cleanliness rating of 5.7 (10 perfect). In contrast, when the same stock was made doctor-sweet by the more costly, copper sweetening process using a single caustic soda prewash, 70 lbs./ 1000 bbls. of the same oxidation inhibitor were required to produce the same stability, and the thus-sweetened gasoline had an Flr-2 test total engine cleanliness rating of only 66 and a piston cleanliness rating of only 3.9.

Numerous modifications and variations of the invention as described will readily suggest themselves to those skilled in the art. Accordingly, the invention should not be limited to the embodiments described but only by the scope of the appended claims.

I claim:

1. A process of sweetening and stabilizing a previously unsweetened, mercaptan-containing light hydrocarbon oil distillate, comprising intimately contacting said distillate concurrently with a small amount of oxygenfat least chemically sufficient to oxidize the mercaptan Sulfur in the oil, and with a partly spent, aqueous, lead suliide-containing, sodium plumbite solution, separating sweetened oil and partly spent sodium plumbite solution, and improving the stability of the thus-separated oil by intimately contacting the same with a second aqueous sodium plumbite solution that is relatively free from lead sulfide and that is of greater strength than said partly spent solution, separating sweetened oil having improved stability characteristics from said second sodium pluinbite solution, and replacing at least a portion of said partly spent sodium plumbite solution with alike amount of said second sodium plumbite solution, and replacing the amount of said second sodium plumbite solution so withdrawn with `a like amount of fresh sodium plumbite solution of the original strength that has not previously been contacted with said oils.

2. A process of sweetening and stabilizing a previously unsweetened, mercaptan-containing, light hydrocarbon oil distillate, comprising intimately contacting said distillate concurrently with a small amount of oxygen, at least Chemically sutcient to oxidize the mercaptan sulfur in the oil, said amount being in the range of about 0.15 to 1.0 cubic foot of air per barrel of said distillate, and with about 10 to 100 percent by volume ot said distillate of a partly spent, aqueous, lead sulfide-containing sodium plumbite solution, said partly spent sodium plumbite solution containing about 4 to 24 percent by weight sodium hydroxide and about 0.5 to 5 percent by weight lead oxide, separating sweetened oil and partly spent sodium plumbite solution, and improving the stability of the thusseparated oil by intimately contacting the same with about l0 to lGO percent by volume of a second aqueous sodium plumbite solution that is relatively free from lead sulfide and that is of greater strength than said partly spent solution, said second sodium plumbite solution containing about 7 to 40 percent by weight sodium hydroxide and about 1 to 5 percent by weight lead oxide, separating sweetened oil having improved stability characteristics from said second sodium plumbite solution, and replacing a portion of said partly spent sodium plumbite solution `with a like amount of said second sodium plumbite solution, such replacement being effected at the rate of about 5 to 25 percent by volume of said partly spent solution per day, and replacing the amount of said second sodium plumbite solution so withdrawn with a like amount of fresh sodium plumbite solution of the original strength that has not previously been contacted with said oils.

3. A process of sweetening and stabilizing a previously unsweetened, mercaptan-containing, light hydrocarbon oil distillate, comprising intimately contacting said distillate concurrently with a small amount of oxygen, at least chemically sufficient to oxidize the mercaptan sulfur in the oil, said amount being in the range of about 0.15 `to 1.0 cubic foot of air per barrel of said distillate, and with about l0 to 100 percent by volume of said distillate of a partly spent, aqueous, lead sultide-containing sodium plumbite solution, said partly spent sodium plumbite solution containing about 4 to 24 percent by Weight sodium hydroxide and about 0.5 to 5 percent by weight lead oxide, said contacting being carried out for a period of time in the range of about a few minutes to eight hours at a temperature in the range of about 60 to F., separating sweetened oil and partly spent sodium plumbite solution, and improving the stability of the thus-separated oil by intimately contacting the same with about 10 to 100 percent'by volume of a second aqueous sodium plumbite solution that is relatively free from lead sulfide and that is of greater strength than said partly spent solution, said second sodium plumbite solution containing about 7 to 40 percent by weight sodium hydroxide and about 1 to 5 percent by weight lead oxide, said contacting being carried out for a period of time in the range of about a few minutes to eight hours at a temperature in the range of about 60 to 175 F., separating sweetened oil having improved stability characteristics from said second sodium plumbite solution, and from time to time replacing a portion of said partly spent sodium plumbite solution with a like amount of said second sodium plumbite solution, such replacement being eilected at the rate of about to 25 percent by volume of said partly spent solution per day, and replacing the amount of said second sodium plumbite solution so withdrawn with alike amount of fresh sodium plumbite solution of the original strength that has not previously been contacted with said oils.

4. A process of sweetening and stabilizing a previously unsweetened, mercaptan-containing, hydrocarbon distillate boiling in the gasoline range, comprising intimately contacting said distillate concurrently with a small amount of oxygen, at least chemically sufficient to oxidize the mercaptan sulfur in the oil, said amount being in the range of about 0.15 to 1.0 cubic foot of air per barrel of said distillate, and with about to 100 percent by volume of said distillate of a partly spent, aqueous, lead sulfide-containing sodium plumbite solution, said partly spent sodium plumbite solution containing about 4 to 24 percent by weight sodium hydroxide and about 0.5 to 5 percent by weight lead oxide, separating sweetened oil and partly spent sodium plumbite solution, and improving the stability of the thus-separated oil by intimately contacting the same with about 10 to 100 percent by volume of a second aqueous sodium plumbite solution that is relatively free from lead sulde and that is of greater strength than said partly spent solution, said second sodium plumbite solution containing about 7 to 40 percent by weight sodium hydroxide and about 1 to 5 percent by weight lead oxide, separating sweetened oil having improved stability characteristics from said second sodium plumbite solution and from time to time replacing a portion of said partly Spent sodium plumbite solution, said portion being in the range of about 5 to 25 percent by volume ot the partly spent solution when the oxidation inhibitor response of the sweetened and stabilized gasoline distillate reaches an objectionable level, with a like amount of said second sodium plumbite solution, and replacing the amount of said second sodium plumbite solution so withdrawn with a like amount of fresh sodium plumbite solution of the original strength that has not previously been contacted with said oils.

5. A process of sweetening and stabilizing a previously unsweetened, mercaptan-containing, hydrocarbon distillate boiling in the gasoline range, comprising intimately contacting said distillate concurrently with a small amount of oxygen, at least chemically sufficient to oxidize the mercaptan sulfur in the oil, said amount being in the range of about 0.15 to 1.0 cubic foot of air per barrel of said distillate, and with about 25 to 60 percent by volume of said distillate of a partly spent, aqueous lead sulfide-containing sodium plumbite solution obtained Jfrom the subsequent treatment of the oil, said partly spent sodium plumbite solution containing about 4 to 24 percent by weight sodium hydroxide and about 0.5 to 5 percent by weight lead oxide, sai-d contacting being carried out for a period of time in the range of about a few seconds to eight hours at a temperature in the range of about to 120 E., separating sweetened oil and partly spent sodium plumbite solution, and improving the stability of the thus-separated oil by intimately contacting the same with about 25 to 60 percent by volume of a second aqueous sodium plumbite solution that is relatively free from lead sulfide and that is of greater strength than said partly spent solution, said second sodium plumbite solution containing about 7 to 40 percent by weight sodium hydroxide and about 1 to 5 percent by weight lead oxide, said contacting being carried out for a period of time in the range of about a few seconds to eight hours at a temperature in the range of about 60 to 120 F., separating sweetened oil having improved stability characteristics from said second sodium plumbite solution and replacing a portion of said partly spent sodium plumbite solution, said portion being in the range of about 5 to 25 percent, by Volume of the partly spent solution when the oxidation inhibitor response of the sweetened and stabilized gasoline distillate reaches an objectionable level, with a like amount of said second sodium plumbite solution, and replacing the amount of said second sodium plumbite solution so withdrawn with a like amount of fresh sodium plumbite solution of the original strength that has not previously been contacted with said oils.

References Cited in the tile of this patent UNITED STATES PATENTS 1,863,967 Burruss June 21, 1932 2,356,704 Shmidl et al Aug. 22, 1944 2,684,926 Brandel July 27, 1954 2,768,931 Ambler Oct. 30, 1956 OTHER REFERENCES Kalichevsky et al.: Petroleum Refining With Chemicals, Elsevier Publishing Company, New York (1956), pages 282-295. 

1. A PROCESS OF SWEETENING AND STABILIZING A PREVIOUSLY UNSWEETENED, MERCAPTAN-CONTAINING LIGHT HYDROCARBON OIL DISTILLATE, COMPRISING INTIMATELY CONTACTING SAID DISTILLATE CONCURRENTLY WITH A SMALL AMOUNT OF OXYGEN, AT LEAST CHEMICALLY SUFFICIENT TO OXIDIZE THE MERCAPTAN SULFUR IN THE OIL, AND WITH A PARTLY SPENT, AQUEOUS, LEAD SULFIDE-CONTAINING, SODIUM PLUMBITE SOLUTION, SEPARATING SWEETENED OIL AND PARTLY SPENT SODIUM PLUMBITE SOLUTION, AND IMPROVING THE STABILITY OF THE THUS-SEPARATED OIL BY INTIMATELY CONTACTING THE SAME WITH A SECOND AQUEOUS SODIUM PLUMBITE SOLUTION THAT IS RELATIVELY FREE FROM LEAD SULFIDE AND THAT IS OF GREATER STRENGTH THAN SAID PARTLY SPENT SOLUTION, SEPARATING SWEETENED OIL HAVING IMPROVED STABILITY CHARACTERISTICS FROM SAID SECOND SODIUM PLUMBITE SOLUTION, AND REPLACING AT LEAST A PORTION OF SAID PARTLY SPENT SODIUM PLUMBITE SOLUTION WITH A LIKE AMOUNT OF SAID SECOND SODIUM PLUMBITE SOLUTION, AND REPLACING THE AMOUNT OF SAID SECOND SODIUM PLUMBITE SOLUTION SO WITHDRAWN WITH A LIKE AMOUNT OF FRESH SODIUM PLUMBITE SOLUTION OF THE ORIGINAL STRENGTH THAT HAS NOT PREVIOUSLY BEEN CONTACTED WITH SAID OILS. 